The present invention relates to a thermoelectrochemical process and apparatus for very efficient, clean, modular, cost-effective, green and climate-change neutral power or combined heat and power generation from biomass. Other carbonaceous feedstocks can be used as well.
Many different options are available for power generation. The fuel can be combusted, gasified, pyrolyzed, bioprocessed or liquefied and utilized in engines, steam power plants (boiler, steam turbine, etc.), gas turbines, gas and steam power plants, and fuel cells. Among these, the most efficient and environmentally superior route for electric power generation is fuel cells. In a fuel cell stack, the fuel is electrochemically reacted with oxygen (from air)xe2x80x94without combustionxe2x80x94producing electricity and usable heat. Efficiencies in the 55 to 70% range (based on HHV) have been projected for fuel cell power generation. The efficiency gain for the fuel cell is especially significant for the small-scale power (10 kWe to 5 MWe) sector. Gas turbines and combined-cycle units are generally not applicable to this size range due to low efficiency and high cost. Here, the traditional steam power plants are generally less than 20% efficient. Engines are more efficient (20 to 40%) but are typically fired with diesel or natural gas. A fuel cell can, however, achieve between 35 and 55% efficiency. Also, the efficiency of the fuel cell stack remains the same regardless of power level and this is an additional advantage over conventional power generation systems.
Many power plants based on biomass combustion have experienced operational difficulties, especially when firing non-wood biomass fuels. These resulted from the deposition of mineral matter on heat exchange surfaces (boiler tubes, superheaters and water walls) or from the agglomeration of ash in a fluidized bed. Gasification of biomass, in contrast, renders it possible to avoid these problems, minimize emissions and integrate with the fuel cell.
Currently, there exists many different gasifiers, such as high pressure, low pressure, partial oxidation or autothermal, indirectly heated, oxygen/air/steam-blown, fixed/fluidized bed or entrained flow gasification. Each system has its advantages. In contrast to prior systems, however, the pulse combustor steam-reforming technology of the present invention, generates a hydrogen-rich, medium-Btu gas (does not need oxygen input) that is well-suited for fuel cell applications.
In direct gasification, partial oxidation or autothermal reactions are employed which yield inferior partial pressure concentrations of the fuel gas for the fuel cell stack (H2 in the case of phosphoric acid fuel cells and H2 and CO in the case of molten carbonate and solid oxide fuel cell stacks). This is due to the fact that both exothermic and endothermic reactions take place in situ in the case of direct gasification, and the products of exothermic reactions dilute the product gases to be consumed by the fuel cell.
In view of the above, currently, a need exists for a new gasification process that is better suited for power generated applications involving the use of a fuel cell.
In accordance with the present invention, a process for producing electricity from carbonaceous materials is disclosed. The carbonaceous materials can be, for instance, coal, pulp and paper waste, wood products such as wood chips or sawdust, municipal waste, industrial waste, sewage, food waste, plant matter, rice straw, black liquor and animal waste.
The process includes. providing a fluidized bed containing a particulate material and a fluidizing medium. The fluidizing medium is steam. The particulate material can have a particle size less than about 500 microns and can include sand, alumina, magnesium oxide, an alkali carbonate, carbon, and the like.
Any suitable combustion device can be used to indirectly heat the fluidized bed. In one embodiment, a pulse combustion device combusts a fuel source to form a pulse combustion stream. The pulse combustion stream indirectly heats the fluidized bed. As used herein, indirectly heating the bed means that the pulse combustion stream does not contact the contents of the bed.
A carbonaceous material is fed to the fluidized bed. The fluidized bed is maintained at a temperature sufficient for the carbonaceous materials to endothermically react with the steam to form a product gas stream. The product gas stream can contain, for instance, lower molecular weight hydrocarbons. The product gas stream is then fed to a fuel cell. The fuel cell can include an electrolyte which chemically reacts with the product gas stream to generate electricity.
Any suitable fuel cell can be used in the process of the present invention. Particular fuel cells include a phosphoric acid fuel cell, a molten carbonate fuel cell, or a solid oxide fuel cell.
The temperature in the fluidized bed can be from about 900 degrees F to about 1800 degrees F. and particularly from about 1100 degrees F to about 1600 degrees F. The carbonaceous materials can remain in the bed for a time from about xc2xd hour to about 15 hours, and particularly from about two hours to about 10 hours. For most applications, the weight ratio between steam and the carbonaceous materials can be from about 0.75:1 to about 3:1.
In order to conserve energy, in one embodiment, a portion of the product gas stream is fed to a heat exchanger that heats steam which is fed to the fluidized bed. Steam can also be generated or heated using the flue gas from the pulse combustion device.
The flue gas of the pulse combustion device can also be used to heat air being fed to the fuel cell or pulse combustion device and can be used to heat or generate steam fed to a dryer for drying the carbonaceous materials prior to being fed to the fluidized bed.
In order to clean the product gas stream prior to being combusted in the gas turbine, the product gas stream can be fed through a cyclone for removing particulate material and can be fed to a scrubber and/or gas polisher for removing any undesirable constituents depending on the fuel cell being utilized.